Methods of multiple stage evaporation from heat sources other than steam

ABSTRACT

A method of multiple stage evaporation incorporating direct contact evaporation, with the flue gas utilized to heat water, the heated water utilized to heat liquor for flash evaporation to a vacuum. The condensed vapors are then used to heat liquor that is air contact evaporated to provide 3 stages of evaporation. Low-quality flue gas heat is also used to heat liquor for air contact evaporation.

Mateo? tates atent Farin 1 Eeh.1,11972 [54] METHODS OF MULTIPLE STAGEEVAPORATEGN FROM HEAT SOURCES OTHER THAN STEAM [72] lnventor: William G.Farin, 707 Elm St., Neenah,

,Wisr54956 l. v.

[73] Assignee: Marathon Engineering Inc., Menaska,

, Wis.

[22] Filed: June 11, 1969 [21] Appl No.: 832,399

[52] US. Cl. ..159/47 WL, 159/4 VM, 159/4 A [51] Km. Cl. "Bind 1/00,B01d 1/16 [58] Field o'lkarch ..159/47 WL,4 A, 13 A, 13C,

159/4 CC, 4 VM, 16 A, 16 B, 16; 55/93, 94, 73;

[56} References Cited UNITED STATES PATENTS 2,343,027 2/1944 Ramen159748 FEED WATER 5 149 PRODUCT 11/1956 Allen, Jr. ..159/l6 A 2,839,1226/1958 Laquiharre ..159/4 A 2,901,061 8/1959 Hartig ..55/73 3,323,2896/1967 Venemark ..55/73 9/1956 Rauh ..l59/l6A Primary Examiner-NormanYudkoff Assistant Examiner-David Edwards [5 7] ABSTRACT A method ofmultiple stage evaporation incorporating direct contact evaporation,with the flue gas utilized to heat water, the heated water utilized toheat liquor for flash evaporation to a vacuum. The condensed vapors arethen used to heat liquor that is air contact evaporated to provide 3stages of evaporation. Low-quality flue gas heat is also used to heatliquor for air contact evaporation.

3 Cflaims, 2 Drawing Figures 2, la /Q PRODUCT PATENTEDPEB H972 3,638,708

SHEET 1 OF 2 I4 vmaooucr 19' m Pdbiibr 30 7o PRODUCTQ INVENTOR.

SHEET 2 BF 2 PUDOOKQ b Gm INVENTOR.

PATENTED FEB 1 I972 METHODS OF MULTIPLE STAGE EVAPORATION FROM HEATSOURCES OTHER THAN STEAM BACKGROUND OF THE INVENTION The concentrationof chemicals, waste liquors and solids obtained from waste disposal andpollution control processes is a serious problem facing industry andmunicipalities. The destruction of wastes by burning is feasible in manycases when concentration can be economically accomplished. However, theburning process often creates added problems due to the air pollutionencountered.

Suspended solids can normally be economically concentrated inclarifiers, vacuum deckers, screens, centrifuges, or presses. Solublesolids are more difficult to concentrate. The evaporation needed forconcentration is often uneconomical.

A process to handle some of these problems for spent pulp mill liquorsis disclosed and claimed in my prior U.S. Pat. No. 3,425,477 issued Feb.4, 1969, Methods for Heat Recovery In Evaporating and Burning SpentLiquor." The subject invention incorporates many improvements in thesemethods that expand its application to the concentration of allchemicals, waste products and solids at lower capital cost and improvedeconomy of operation.

These new methods enable economical concentration by burning commercialfuel, the concentrated product waste, or a combination thereof tosatisfy evaporation heat requirements, and utilizing the heat generatedin a multiple manner. The very low quality heat normally discharged tothe atmosphere can now be utilized by air contact evaporation.

It addition to utilizing the heat recovered for evaporation andconcentration of waste products, the system enables removal of airpollutants and the thermal pollution normally encountered in thecondensation of water vapor from evaporator operations.

It is the main purpose of this invention to provide a more efficientevaporation process to concentrate soluble waste products and chemicalsfor disposal, sales and pollution abatement.

Another objective of this invention is to provide a more economicalmethod of concentrating solutions by evaporation utilizing heat sourcesother than steam.

Another object of this invention is to provide means of concentratingsoluble waste products utilizing waste heat sources.

Another object of this invention is to prevent air pollution in theevaporation and burning of soluble waste products.

Another object of this invention is to provide more efficientevaporation while eliminating cooling water requirements and thermalpollution from evaporation.

Another object of this invention is to combine oxidation in anevaporation step to reduce air pollution.

SUMMARY OF THE INVENTION In this invention, the flue gas used forevaporation may be generated by burning gas, oil, coal, the concentratedwaste products, concentrated liquor, or any combination thereof toprovide the necessary heat required for evaporation. Firing may be donein a burner, furnace, incinerator, fluidized bed reactor, reductionfurnace, boiler or a recovery boiler.

The hot flue gas generated by such burning may have a portion of theheat utilized for the production of steam or power, with the balance ofthe heat used for evaporation, or all of the heat generated may be usedfor evaporation. Evaporation may be caused to take place in any one, twoor all three of the fol lowing stages:

1. Direct contact evaporation stage 2. Heat recovery vacuum evaporationstage 3. Air contact evaporation stage in the direct contact evaporationstage, the hot flue gas is placed in direct contact with the chemical,waste product or liquor being concentrated and contact is maintained fora sufficient period to cool the flue gas to near its saturationtemperature by evaporation of water from said chemical, waste product orliquor being concentrated.

Direct contact evaporation may be done in a venturi scrubber, submergedcombustion evaporator, spray tower or in a fluidized bed reactor orincinerator where the waste is carried countercurrent to the gas flow sothat it is predried by the flue gas and then burned when driedsufficiently to support combustion.

The flue gas saturated by direct contact evaporation still maintains thefull heat value but such value is in the form of latent heat of thecontained water vapor. This flue gas is then scrubbed and cooled by acirculating stream of water or condensate to recover the heat ofcondensation derived from the reduced saturated water vapor content atlower flue gas temperatures. The water is heated by countercurrent flowof the flue gas to provide the heat media for heat recovery vacuumevaporation. The scrubbing may also be utilized to recover chemicalsreleased by burning and direct contact evaporation and to prevent airpollution.

Scrubbing of the flue gas and concomitant chemical recovery may beaccomplished in three different ways. In the first method, chemicalrecovery is incorporated in the heat recovery stage so that the Water orcondensate heated with or without chemical addition is also utilized torecover chemicals or contaminants from the flue gas. As noted below, aflue gas contaminant such as sulfur dioxide, for instance is removedmore effectively by the addition of an alkaline chemical such as calciumhydroxide. The calcium bisulflte produced is recoverable in solution forreuse as long as the dilution from the condensate produced is notobjectionable.

In the second method, chemicals can be recovered and contaminantsremoved before heat recovery, by a gas phase chemical reaction at peaksaturation temperatures. Flue gas ingredients such as sulfur dioxide,can be removed by scrubbing with water containing alkaline solutionssuch as calcium hydroxide, magnesium hydroxide, sodium hydroxide orsodium carbonate at the peak saturation temperature. This enablesseparation and removal of resulting chemicals such as the calciumsulfite, magnesium sulfite or sodium sulfite produced in the form of asolution or slurry at relatively high concentrations for disposal orrecovery. By removing such chemicals and contaminants by chemicalscrubbing previous to heat recovery, contamination of the condensateproduced in condensing the water vapor in the heat recovery stage isavoided and air pollution prevented.

The third method of scrubbing is done following heat recovery and isused where lower temperatures are desired to facilitate chemicalabsorption and chemical recovery. This latter method is adaptable, forinstance, for recovering sulfur dioxide with ammonia for ammoniabisulfite cooking liquor makeup in an ammonia base pulp mill recoverysystem. The lower temperatures reduce the ammonia losses.

Scrubbing with or without the addition of chemicals such as ammonia maybe carried out in a second stage of heat recovery. Due to the decreasingrate of heat recovery at the lower temperatures, two or more stages ofheat recovery reduce condensate circulation requirements at the lowertemperatures and enable high circulation rates at the highertemperatures. This increases the heat recovery potential of the highertemperature heat, improves heat recovery evaporation temperaturedifferentials available, reduces heat exchanger requirements and permitsmore heat to be utilized for two or more stages of heat recovery vacuumevaporation.

The direct contact scrubbing, heat recovery scrubbing, and chemicalrecovery scrubbing may be done in a series of scrubbers, however,considerable savings are possible by utilizing a single multiple stagescrubber. The flue gas draft may be a forced draft system from the airsupply system to the burner. A suction fan may also be used at the fluegas discharge and in many cases, both will be required. The scrubbersused may be venturi scrubbers, plate towers, packed towers or anyscrubber design to meet individual requirements. For maximum economy,the heat recovery towers should be countercurrent units.

when all the heat generated from a burning operation is utilized fordirect contact evaporation, the saturation temperatures of the flue gasfollowing direct contact evaporation are normally in the 190 F. range.The saturated heat content and water vapor content will vary somewhatwith the flue gas involved, but will be in the vicinity of 1,300 B.t.u.per pound of dry flue gas generated and will carry about l.l pounds ofwater vapor per pound of dry flue gas following direct contactevaporation.

When the flue gas is cooled to 160 F., the heat content is reduced toabout 370 B.t.u. per pound of dry flue gas and the water vapor contentis reduced to 0.3 pounds per pound of dry flue gas. Due to thiscondensation, over 70 percent of the heat is recoverable by using acountercurrent flow of condensate that is being heated from 155 to the185 F. range. This condensate then becomes the heating media for heatrecovery vacuum evaporation.

The condensate is then utilized in a heat exchanger to heat acirculating flow of the liquor being concentrated and which isevaporated at lower temperature and pressure conditions induced byvacuum equipment. For liquor evaporated in the l20 to 140? F.temperature range there is a sufficiently high temperature differentialfor very economical use of the heat exchanger surface for single effectevaporation and for two ef fect evaporation where conditions warrant.

ln condensing the water vapor in the heat recovery system, advantage istaken of the latent heat of fuels and the high heat values can beutilized. The single effect heat recovery evaporator can usually doublethe evaporation possible by direct contact evaporation along. The lowcost of the heat in using the commercial fuel direct, makes the unitcompetitive in operating heat costs with a four effect steam evaporatorof equal evaporation capacity. When burning a disposal product, theseheat costs for evaporation are eliminated entirely.

The heat recovery evaporator used may be a spray film, falling film,rising film, natural circulation, forced circulation, reverse cyclescale prevention or flash evaporator of a single or multiple effect. Itcan also be followed by an air contact evaporator utilizing the heat ofthe vapors produced to provide added evaporation, eliminate coolingwater requirements and the thermal pollution normally encountered fromevaporation systems.

The evaporation taking place in the heat recovery evaporator is carriedout under vacuum conditions at relatively low evaporation temperatures.This reduces some of the scaling, polymerization, corrosion and productbreakdown sometimes encountered in the atmospheric boiling temperaturerange.

if the vapors released by the heat recovery evaporator are contaminatedwith chemicals such as sulfur dioxide, they can be chemically scrubbedor neutralized in the vapor phase with water containing alkalinesolutions such as calcium hydroxide in the same manner as the gas phasetreatment of the flue gas at peak saturation temperatures previouslymentioned. The ordinary single effect heat recovery step requires onlyone treatment and collection, while multiple effect evaporators requiretreatment at every stage. These contaminants can also be controlled insome cases by chemical pretreatment of the feed liquor with calciumhydroxide for instance so the contaminants such as sulfur dioxide arenot released during evaporation.

The vapors released by the heat recovery evaporator or any otherevaporator ordinarily require cooling water to condense the vaporseither by direct contact in a barometric condenser or in a surfacecondenser. If this heat can not be utilized, the heated water becomes asource of thermal pollution when returned to the stream.

The cooling water requirements and the thermal pollution involved areeliminated by an air contact evaporator that cools the liquor beingconcentrated in it, so that it can be used as the cooling media in thesurface condenser of the heat recovery vacuum evaporator. The liquorcooled to the 90 to I F. range by air contact evaporation is heated tothe 120 to 125 F. range by the condensing vapors in the surfacecondenser. The heated liquor is then contacted countercurrently by aflow of air that is heated by the liquor. The heated air becomessaturated with water vapor picked up from the liquor and is dischargedat temperatures in the 115 F. range. The liquor is cooled by the air andby the evaporation down to the to l00 F. range where it is again usedfor cooling in the said surface condenser.

in the air contact evaporator. a pound of water is evaporated for every1,200 to 1,600 B.t.u. of heat picked up in the surface condenserdepending on atmospheric conditions. The B.t.u. requirements are thelowest in the summer and more air contact evaporation is possible. Inthe winter, when the B.t.u. requirements are the highest and air contactevaporation rates are reduced, the temperatures differentials availableare higher and more heat can be recovered. This increases both the heatrecovery evaporation and air contact evaporation to satisfy theincreased winter heat demand and equalizes year round evaporation rates.

The air contact evaporator may also utilize the heat in the flue gasthat can only be used at lower temperatures. Using air contactevaporation, the flue gas can be cooled to the 1 l0 to l20 F. range andover 90 percent of the heat following direct contact evaporation can beused for air contact evaporation. in cooling the flue gas, it permitslower temperature scrubbing without added cooling water and thermalpollution. The recovery of chemicals and air pollution control is alsoimproved at these lower temperatures.

The oxidation made possible by air contact evaporation is also of primeimportance in some operations. For Kraft pulp mill spent black liquor,oxidation ties down sulfur compounds that would otherwise be released indirect contact evaporation and burning. The oxidation prevents the airpollution which otherwise would be involved and enables the recovery ofthese sulfur compounds otherwise lost to the atmosphere.

The air contact evaporation equipment may be a cooling tower, scrubber,trickel tower, oxidation tower, spray tower or any other towerperforming one of these functions. It can be a natural draft tower orhave forced or induced fans to provide air handling capacity.

The overall evaporation system incorporates direct contact evaporation,heat recovery vacuum evaporation and air contact evaporation using thesame heat for each operation step. However, any one or two of thesesteps can be eliminated when waste heat is being utilized or when onlylimited evaporation is required.

The feed system can be varied to meet individual product requirements.Final concentration will be done in the direct contact evaporator whenpossible to enable handling liquor at lower concentration andviscosities in the vacuum and air contact evaporators. This willminimize heat transfer surface requirements and facilitate handling.When scaling, polymerization, carbonization or corrosion is encounteredat the high concentration and temperature of direct contact evaporation,the final concentrating can be carried out in a heat recovery vacuumevaporator.

The chemicals released in burning, and picked up by the liquor duringdirect contact evaporation, may also influence the feed system. Forinstance, the calcium oxide in the fly ash released in burning pulp millcalcium base sulflte liquor will be picked up by the liquor duringdirect contact evaporation and tie down the organic acids and sulfurdioxide that would otherwise be released in evaporation. If, in the feedsystem, direct contact evaporation precedes vacuum evaporation, theliquor is thereby treated before vacuum evaporation and contamination ofthe released vapors and formed condensate is avoided. When the calciumbase liquor is burned, the organic acids are decomposed and the sulfurdioxide is released and recovered by the flue gas scrubbing system.

Where the flue heat content is low, the heat recovery vacuum evaporationstep can be eliminated and the direct and air contact systems utilizedfor evaporation. Following direct contact evaporation the heatedcondensate is used directly, in this case, to heat the liquor beingconcentrated and cooled by air contact evaporation. Where the moisturecontent of flue gas is already high, the direct contact evaporation stepcan be eliminated. Heat recovery vacuum evaporation can still becombined with air contact evaporation or either evaporation system usedseparately.

This permits a very versatile and economical use of heat discharged fromthe normal boiler, recovery boiler, direct contact evaporators, gasturbine or incinerator for evaporation purposes. It also enablesscrubbing the air for pollution control and preventing thermal pollutionfrom the evaporation process.

Various objects and advantages of the invention will become apparent topersons skilled in the art upon examination of the specifications andaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a schematic flow diagram showing direct contact evaporation,heat recovery vacuum evaporation and air contact evaporationincorporating the present invention in conjunction with a burner.

FIG. 2 is a schematic flow diagram showing direct contact evaporationand air contact evaporation incorporating the present invention inconjunction with a recovery boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the arrangement shown inFIG. 1, fuel is burned in a burner 1, utilizing air from fan 2, that ispreheated in the outside buming chamber 3. The air contacts the fuelfrom fuel nozzle 4 to support the combustion that takes place inburner 1. The resulting hot flue gas passes into the venturi scrubber 5,where it contacts a circulating stream of the liquor being concentratedas it is supplied through the distributor ring 6. Evaporation from theliquor causes the liquor to become more concentrated, drops thetemperature of the flue gas to the 190 F. range, and saturates the fluegas with water vapor as it enters chamber 7 of the first stage of thescrubber.

The circulating liquor concentrated by the direct contact evaporation iscollected in collection chamber 8 and discharged through line 9 to pump10. The liquor is returned through line 11 to the distributor ring 6 tocomplete the circulation cycle of the liquor being concentrated bydirect contact evaporation.

Following this direct contact evaporation stage, the flue gas passesthrough separator 12 to prevent liquor carry through and goes to thesecond or chemical scrubbing stage 13. Chemical such as calciumhydroxide, magnesium hydroxide, sodium hydroxide, sodium carbonate andammonium hydroxide may be added to water introduced at inlet 14 intochemical scrubber stage 13 at 190 F and remove contaminants such assulfur dioxide from the flue gas that might otherwise be carried out theflue gas discharge 15 to pollute the air or be discharged with thecondensate through the condensate outlet 16. The chemicals andcontaminants removed from the flue gas in the form of calcium sulfite,magnesium sulfite, sodium sulflte or ammonia sulfite solution or slurryby scrubbing in the second stage 13 of the scrubber are dischargedthrough outlet 17 at high concentration for recovery or disposal.

Following chemical scrubbing, the flue gases, still at 190 F., continueupward through separator 18 to prevent carry through of chemicals intothe third or heat recovery stage 19 of the scrubber. In the heatrecovery stage 19, the flue gas is cooled to 160 F. by a circulatingflow of condensate introduced into the scrubber through line 20 at 155F. and discharged through line 21 at 185 F.

The heated condensate from the heat recovery scrubber stage 19, passesout line 21, through pump 23 and line 24 to heat exchanger 25. In heatexchanger 25, the condensate is cooled from 185 to 155 F. by a secondstream of circulating liquor being concentrated by heat recovery vacuumevaporation. The cooled condensate is returned through line 20 to theheat recovery scrubber stage 19.

Chemicals such as calcium hydroxide may also be added to the condensatethrough inlet line 72 to remove contaminants such as sulfur dioxide anddischarge the calcium bisulfite solution produced through outlet 16.However, the water vapor content of the flue gas is reduced by over 70percent in being cooled from 190 to 160 F. The resulting excesscondensate released in the heat recovery stage 19 is also dischargedthrough outlet 16 and concentrations are reduced.

The flue gas leaving chamber 19 at 160 F. passes to chamber 26 and iscooled to 1 15 F. and discharged through separator 27 and outlet 15.Water enters through inlet line 28 at F. to cool the flue gas and isheated to 155 F. and discharged through line 29, pump 30 and line 31 toheat exchanger 32 where it is cooled to 1 10 F. The cooled liquor isreturned to chamber 26 through line 28. Chemicals such as calciumhydroxide may also be added through inlet 34 to the condensate to removecontaminants such as sulfur dioxide and discharge the calcium bisulfltesolution produced with excess condensate through outlet 35.

The circulating liquor stream entering heat exchanger 25 through inletline 36 is heated from 141 to 146 F. and is discharged through line 37into the heat recovery vacuum evaporator 38. Here the liquor is cooledby evaporation from 146 to 141 F. and is carried through funnel 39, line40, pump 41 and line 36 back to heat exchanger 25 to complete thecirculation cycle of the liquor being concentrated by the heat recoveryvacuum evaporator.

The vapors released in the vacuum evaporator chamber 38 at 140 F. arescrubbed in the treatment chamber 412 by chemicals such as calciumhydroxide added through inlet 43, to remove contaminants such as sulfurdioxide to produce a calcium sulflte solution or slurry that isdischarged through outlet 44. The vapors pass up through separator 45 toremove and prevent carryover of entrained chemicals such as the calciumsulfite produced with the vapors through outlet 46 to the surfacecondenser 47 where the vapors are condensed. The entrained chemicals areheld in chamber 42 and are removed through outlet 44. The condensate isremoved through outlet 48 and the noncondensables drawn through line 49by the vacuum eductor 50 utilizing steam at inlet 51. The steam andnoncondensables are discharged through line 52 to. the first stagescrubber chamber 7 for chemical recovery in stage 13 and heat recoveryin stages 19 and 26.

The heat of condensation in the surface condenser 47 is used to heat athird circulating flow liquor being concentrated in this case by aircontact evaporation. This liquor enters the surface condenser 47 throughinlet line 53 and is heated from 95 to 125 F. by condensing vapors at140 F. The heated liquor is discharged through line 54 to air contactevaporator 55 and enters through spray nozzles 56.

Air enters the air contact evaporator 55 through louvers 57 and flowscountercurrent to the sprayed liquor and is heated thereby fromatmospheric temperature to F. The air, saturated with water vapor pickedup from the liquor, is discharged through outlet 58. The liquor, cooledto 95 F. and concentrated due to the water removed, drops to thecollection pan 59. The liquor is then discharged through line 60 to pump61 and lines 62 and 53 to heat exchanger 47 to complete a circulatingloop of a portion of the liquor being concentrated by air contactevaporation.

The remainder of the liquor flow from line 62 that does not go throughline 53 goes through line 64 to heat exchanger 32 where it is heatedfrom 95 to F. The liquor is discharged from heat exchanger 32 throughline 65 to line 54, joins the flow of liquor to the air contactevaporator 55 entering through spray nozzles 56.

The evaporation is thus carried out in three stages using the heat firstfor direct contact evaporation at scrubber stage '7, then for heatrecovery vacuum evaporation at evaporator chamber 38 and again for aircontact evaporation in air contact evaporator 55.

The feed system can be varied to meet individual requirements. FIG. 1shows 10 percent feed liquor entering through line as to the air contactevaporation circulating system at line 54. The concentrated product at14 percent solids is discharged from line 62 through line 67 and entersthe direct contact evaporation system at line 9. The concentratedproduct, after direct contact evaporation to 19 percent solids, isdischarged through line 68, pump 69 and line 70 to enter heat recoveryevaporation system at line 37. The final product concentrated to 30percent solids by vacuum evaporation is discharged through line 71. Thefeed may also go to (l) vacuum evaporation, (2) air contact evaporation,and (3) direct contact evaporation or varied in any other manner desiredto meet individual needs.

In the arrangement shown in FIG. 2, concentrated product such as spentpulp mill digester liquor at 60 percent solids is burned in a boiler 101at nozzles 102 feed water is added through line 149, heated in anindirect coil 150 and steam is generated through outlet 103 with theflue gas discharged through outlet 104 at a temperature in the 440 F.range. The flue gases contact a circulating stream of liquor beingconcentrated by direct contact evaporation entering at distributor ring105 and direct contact evaporation occurs in the venturi 106 of thescrubber first stage chamber 107.

The liquor concentrated by direct contact evaporation drops to thecollection chamber 108 and is discharged through lines 109 and 110 topump 111 and through line 112 back to the distributor ring 105 tocomplete the circulating loop of liquor being concentrated by directcontact evaporation.

The flue gas is cooled by direct contact evaporation to a saturationtemperature of about 162 F. as it enters the first scrubber stagechamber 107, then passes through separator 113 to prevent liquor carryover into the heat recovery scrubber chamber 114. In the heat recoveryscrubber chamber 114, the flue gas is cooled to 139 F. by acountercurrent flow of condensate that enters through line 1 at 134 F.and is heated to 157 F. and discharged through line 116 to pump 117 andthrough line 118 to heat exchanger 119. In heat exchanger 119, thecondensate is cooled from 157 to 134 F. and circulated back to the heatrecovery scrubbing chamber 114 through line 115. The excess condensatecondensed in the heat recovery scrubbing chamber 114 is discharged fromthe circulating system through outlet line 120 to the pulp mill chemicalrecovery system.

The flue gas at 139 F. following heat recovery in chamber 114 moves tothe third or chemical scrubbing chamber 121 where it is cooled from 139to 1 15 F. by a countercurrent flow of chemical scrubbing liquor heatedcountercurrently from 1 10 to 134 F. The cooled and scrubbed flue gaspasses through separator 122 to remove entrained chemicals and isdischarged through outlet line 123 by fan 124 to the atmosphere.

The heated chemical scrubbing liquor at 134 F. is removed from thechemical scrubbing chamber 121 through line 125 to pump 126 and throughline 127 to heat exchanger 128. In heat exchanger 128, the scrubbingliquor is cooled from 134 to 110 F. and is returned to the chemicalrecovery scrubbing chamber 121 through line 129 completing the chemicalscrubbing circulating loop. Scrubbing chemicals such as aqua ammonia areadded through inlet line 130 to remove sulfur dioxide from the flue gasand the ammonia bisulfite recovered and excess condensate are dischargedthrough outlet line 131.

The heat is removed from heat exchangers 119 and 128 by circulatingliquor being concentrated by air contact evaporation. Liquor cooled byair contact evaporation to 90 F. enters heat exchanger 128-through inletline 132 and is heated to 100 F. and is discharged through line 133 toheat exchanger 119. In heat exchanger 119, the liquor is heated to 125F. and

.is discharged through lines 134 and 135 to air contact evaporator 136and distributed by spray nozzles 137.

Air enters the air contact evaporator 136 through louvers 138, flowscountercurrent to the sprayed liquor and is heated thereby fromatmospheric temperature to 115 F. The air, saturated with water vaporreleased by the liquor, is discharged through outlet 139. The liquor,cooled to 90 F.

and concentrated due to the w ater removal, drops to collection pan 140.The liquor is then discharged through line 141 to pump 142 and throughline 132 to heat exchanger 128 to complete the circulating loop ofliquor being concentrated by 5 air contact evaporation.

Feed liquor at 8 percent concentration is supplied through line 143 toline 135 of the air contact evaporation circulating system. Liquorconcentrated to 18 percent solids by air contact evaporation isdischarged from line 132 through line 144 and enters the direct contactevaporation circulating loop at line 110. Liquor concentrated to 60percent by direct contact evaporation is discharged from line 109through line 145 to pump 1416 and through line 147 to join the 60percent product auxiliary supply line 148 to the boiler burners 102.

A variety of equipment may be utilized in the methods defined in thisinvention. The burning equipment, direct contact evaporation equipment,heat and chemical recovery scrubbing equipment, heat recovery vacuumevaporation equipment and the air contact evaporation equipment can allbe varied within the methods defined in this invention.

While several and specific embodiments of the inventive concept havebeen set forth herein, it is understood that the invention is not to beconstrued as limited thereby and that suitable modifications andvariations may be made without departing from the spirit and scope ofthe invention as defined in the appended claims.

What 1 claim is:

1. A method for more efficiently using heat from flue gas generated byburning than is normally attained by direct contact evaporation whichcomprises providing a first circulating flow of preconcentrated liquorto be further concentrated, directly contacting said circulating liquorwith hot flue gases, maintaining said flue gas in contact with saidliquor for a sufficient period to raise the moisture content of the fluegas to near its saturation point while causing heat to transfer betweensaid hot flue gases and said circulating liquor and concentrate saidcirculating liquor, subsequently cooling said hot flue gases by a seconddirect contact with a circulating water stream whereby the water isheated and the heat of condensation of the gas borne saturated vapor isrecovered in the heated and collected water stream at the reducedsaturation point of the cooled flue gas; providing a second circulatingflow of preconcentrated liquor to be further concentrated in a flashevaporator, circulating the heated water stream resulting from saidsecond direct contact through an indirect heat exchanger to heat saidsecond circulating flow of liquor to be concentrated, inducing a vacuumin said second circulating flow of liquor to flash evaporate and furtherconcentrate said second flow of liquor at a pressure and temperaturebelow the pressure and temperature to which said second flow was raisedin said heat exchanger, cooling and condensing the vapors from the flashevaporator of said second liquor flow in an indirect heat exchanger witha third flow of preconcentrated liquor, thereby heating the said thirdflow of liquor, directly contacting the heated third flow of liquor witha flow of air, maintaining contact for a sufiicient period to cause heatand water vapor to transfer from the said third flow of liquor to saidflow of air, raising the moisture content of said flow of air to nearits saturation point and thereby cooling and concentrating said thirdflow of liquor.

2. The method set forth in claim 1, wherein the said flue gas cooled bydirect contact with said circulating water stream are further cooled bya third direct contact with a second circulating water stream wherebythe said second water stream is heated and the heat of condensation ofthe gas borne saturated vapor recovered in the heated and collectedwater stream at the reduced saturation point of the cooled flue gas;circulating the said second water stream resulting from said thirddirect contact through an indirect heat exchanger, cool ing the saidsecond water stream with a portion of the flow of the said third liquorflow of liquor being concentrated in said indirect heat exchanger,thereby heating a portion of said third flow of liquor beingconcentrated for air contact 5 evaporation.

3. The method set forth in claim 1, wherein the said flue gas withmoisture content raised to near its saturation point following directcontact evaporation with the first flow of liquor being concentrated, iscontacted with a separate circulating water stream at near the flue gassaturation temperature to remove contaminants before said subsequentcooling.

2. The method set forth in claim 1, wherein the said flue gas cooled bydirect contact with said circulating water stream are further cooled bya third direct contact with a second circulating water stream wherebythe said second water stream is heated and the heat of condensation ofthe gas borne saturated vapor recovered in the heated and collectedwater stream at the reduced saturation point of the cooled flue gas;circulating the said second water stream resulting from said thirddirect contact through an indirect heat exchanger, cooling the saidsecond water stream with a portion of the flow of the said third liquorflow of liquor being concentrated in said indirect heat exchanger,thereby heating a portion of said third flow of liquor beingconcentrated for air contact evaporation.
 3. The method set forth inclaim 1, wherein the said flue gas with moisture content raised to nearits saturation point following direct contact evaporation with the firstflow of liquor being concentrated, is contacted with a separatecirculating water stream at near the flue gas saturation temperature toremove contaminants before said subsequent cooling.